Lean NOx Trap Catalyst Research Articles

Control oriented model of a lean NOx trap for the catalyst regeneration in a 2.2 L direct injection diesel engine

In this study a control oriented lean NOx trap (LNT) model was successfully developed for the LNT regeneration purpose to estimate the NOx storage fraction, NOx concentration out of an LNT catalyst and an LNT catalyst bed temperature. The LNT model consists of storage, reduction and 0-D catalyst bed temperature submodel. The model parameters were estimated by the least square fit method in the environment of MATLAB and Simulink. This model was validated through transient engine data to simulate the NEDC mode. The NOx storage fraction in the LNT catalyst can be used as a mean of the determination of the LNT regeneration timing and duration control. The catalyst bed temperature model monitors the thermal behavior. The LNT model was validated by the NEDC mode data shows less than 7% error of cumulated NOx out amount during the storage phase. In addition the catalyst bed temperature model exhibits quite similarly to the experimental data. Therefore the LNT model developed in this study can be applied to the vehicle application as an LNT regeneration control.

2.2L 직분사 디젤 엔진에서 LNT 촉매 재생을 위한 환원제 분사 방법 비교

In this study we investigated the regeneration methods for the lean NOx trap (LNT) catalyst in a 2.2L direct injection diesel engine. The regeneration methods were 1) in-cylinder post fuel injection and 2) external fuel injection strategy. The in-cylinder post fuel injection method uses in-cylinder injectors with the addition of the post fuel injection to supply enough reductants such as CO, H 2, THC. The external fuel injection method was enabled by installing a fuel injector with a wide spray angle before the LNT catalyst. Through the engine experiment, the NOx conversion efficiency, the amount of reductant exhaust gases, fuel consumption, and temperature behavior in the LNT catalyst were evaluated and compared for the two regeneration methods.

NOx storage and reduction properties of model manganese-based lean NOx trap catalysts

In order to study the role of manganese in LNT catalysis, model Pd/Mn/Ba/Al, Pd/Mn/Al and Pd/Ba/Al catalysts were prepared and characterized. Both Mn-containing catalysts exhibited higher activity for NO oxidation to NO2 than the Pd/Ba/Al reference catalyst, although NOx-TPD experiments showed that adsorbed NOx species were weakly bound on Pd/Mn/Al. Consequently, the presence of Ba was essential for high storage capacity. Compared to the Pd/Ba/Al reference, Pd/Mn/Ba/Al showed significantly improved NOx conversion under lean-rich cycling conditions as a consequence of its increased activity for NO oxidation and hence, superior NOx storage efficiency. In addition, the presence of Mn greatly lessened the inhibiting effects of H2O and CO2 on cycle-averaged NOx conversion, this being due to the facile decomposition of manganese carbonate at low temperatures as evidenced by DRIFTS. Significantly, the Pd/Mn/Ba/Al catalyst displayed comparable activity to a traditional LNT catalyst of the Pt/Ba/Al type, showing the promising prospect of such new type of LNT catalysts.

A Highly Active and Stable Non-Platinic Lean NOx Trap Catalyst MnOx-K2CO3/K2Ti8O17 with Ultra-Low NOx to N2O Selectivity

A non-platinic lean NOx trap catalyst MnOx-K2CO3/K2Ti8O17 (K2CO3 loading: 25 wt %) was prepared via successive impregnation, which exhibits a large NOx storage capacity (3.21 mmol/g), a high NOx reduction percentage (98.5%) and an ultralow selectivity of NOx to N2O (0.3%). The catalyst was characterized by multiple techniques including XRD, SEM/HRTEM, EXAFS, FT-IR, CO2-TPD and in situ DRIFTS. Except for K2O, -OK groups, surface K2CO3 and bulk or bulk-like K2CO3, an unknown titanate phase with a K/Ti atomic ratio higher than 2/8 is identified, which is also active and regenerative for NOx storage and reduction. In-situ DRIFTS results reveal that NOx is mainly stored as bidentate nitrates and bidentate nitrite species in the catalyst. The appearance of negative bands around 1555 and 1575 cm–1 (C═O stretching vibration in bidentate carbonates) suggests the involvement of carbonates in NOx storage. Based upon the characterization results, a carbonate-involved NOx storage/reduction mechanism was proposed.

Passive-ammonia selective catalytic reduction (SCR): Understanding NH3 formation over close-coupled three way catalysts (TWC)

NH3 formation was examined under steady-state and lean/rich cycling conditions over four commercial catalysts including: (1) a Pd-only, high precious metal loading (HPGM) three-way catalyst, (2) a Pd/Rh+CeO2, low precious metal loading (LPGM) three-way catalyst, (3) a combination of a HPGM and a LPGM (Dual-Zone) catalyst and (4) a lean NOX trap (LNT) catalyst. The goal of this work was to evaluate these catalysts for their potential use as the upstream component in a passive-NH3 SCR configuration. NH3 formation during steady-state operation was found to be dependent on the air-to-fuel ratio (AFR), temperature and catalytic formulation used. While all of the formulations produced significant amounts of NH3 when operated under sufficiently rich conditions, in general the steady-state NH3 yield decreased in the following order: HPGM≥Dual-Zone>>LPGM≈LNT. Under lean-rich cycling conditions that would be required for this mode of operation, lower air-to-fuel ratios were required to generate the same amount of NH3 as under steady-state conditions. Results obtained with the LNT catalyst demonstrated that at moderate temperatures (i.e., 275–500°C) NOX storage capacity significantly increased the amount of NH3 produced in relation to the amount of NOX slipped. Consequently, the addition of an “optimum” amount of NOX storage capacity in addition to well-controlled lean-rich timing, could significantly improve the performance of the three-way catalyst used as the upstream component in a passive-NH3 SCR configuration. When the CO, C3H6 and N2O concentrations in the effluent were considered in addition to the NH3 formation, an optimum temperature of 400–450°C was determined for the operation of these catalysts.

New insights on N2O formation pathways during lean/rich cycling of a commercial lean NOx trap catalyst

Pathways for N2O formation during lean/rich cycling of a commercial LNT catalyst (containing Pt, Pd, Rh as well as Ba, CeZr, MgAl and Al oxides) were investigated based on bench-reactor experiments with spatiotemporally resolved gas measurements (SpaciMS) and in situ surface analysis (DRIFTS). The inlet gas temperature, composition of the rich gas mixture and regeneration length were varied in order to reveal the underlying mechanisms. Particular attention was given to low temperatures where the regeneration was kinetically limited and N2O was emitted in two peaks. The primary N2O peak appeared immediately after rich-phase inception when platinum-group-metal (PGM) sites over which NOx reduction took place were only partially reduced, and tailed off with the breakthrough of the reductant front. The secondary N2O peak appeared at the rich-to-lean transition as a result of reactions between surface-deposited reductive species (NH3, CO and/or isocyanates) and residual stored NOx. Therefore, more thorough regeneration (longer rich phase, more efficient reductant, or higher temperature) led to a reduced secondary N2O peak. In contrast, CO either produced from reverse water gas shift or injected as a feed could poison PGM sites resulting in self-inhibition of the regeneration and a subsequent significant secondary N2O peak.

NOx Adsorption and Reduction over LaMnO3 Based Lean NOx Trap Catalysts

We have reported a LaMnO3-based, Pt free lean NOx trap (LNT) catalyst shows promising NOx reduction activities, but the mechanism of NOx reduction or the role of the perovskite over this catalyst has not been addressed. This paper presented our investigation on the mechanistic aspects of NOx adsorption and reduction over several LaMnO3-based LNT catalysts. NO can be effectively oxidized to NO2 on LaMnO3 at 350 °C, while little NOx (NO + NO2) can be stored verified by the following temperature programmed desorption experiment; under the same conditions, a significant amount of NOx (41 μmol NO/g washcoat) were stored over BaO/Al2O3/LaMnO3 indicating NOx are adsorbed mainly on the BaO/Al2O3. The temperature programmed surface reaction with CO demonstrated that although it can store NOx during the lean period, failed to reduce NOx by CO over the BaO/Al2O3/LaMnO3 catalyst. After adding Pd into BaO/Al2O3/LaMnO3, NOx can not only be stored during the lean conditions but also reduced. It could be concluded that the Pd catalyst, not perovskite, is responsible for NOx reduction during the rich conditions.

LNT–SCR dual-layer catalysts optimized for lean NOx reduction by H2 and CO

Monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of a lean NOx trap (LNT) catalyst were optimized to provide high NOx conversion at both low and high temperatures with minimal precious group metal (PGM) loading using H2/CO reductant mixtures. The optimized dual-layer catalyst circumvents the need for urea feed and has the potential to reduce the expensive PGM loading by up to 38% from that of LNT only catalyst under laboratory test. We investigated the impact of catalyst design variables, such as SCR and LNT zoning, the ceria level in LNT, as well as SCR zeolite type (ZSM-5, SSZ-13) layer thickness. Zoning of either or both the SCR and LNT in the dual-layer catalysts enables an increase of the low-temperature NOx conversion, and minimizes the high temperature (300–400°C) conversion loss caused by the SCR diffusion resistance and undesired NH3 oxidation by the LNT. High ceria loading of the LNT enhanced NH3 generation, NOx adsorption and mitigated CO poisoning at low temperatures (150–250°C). Commercial Cu-SSZ-13 exhibited a higher NH3 storage capacity and better low-temperature SCR activity than the in-house synthesized Cu-ZSM-5, and improved the low-temperature NOx conversion of the dual-layer catalysts. The diffusion resistance in the top active Cu-zeolite layer inhibited the overall NOx reduction as shown by replacing it with an inert Na-ZSM-5 layer with a high Si/Al ratio. Washcoat diffusion limitations adversely affect the high temperature performance more than the NH3 oxidation to NOx. The experiments revealed that diffusion limitations in the top SCR layer loading of 1.0g/in.3 started at 150°C using a pure H2 feed and at 250°C using a CO/H2 feed.

LNT/CDPF catalysts for simultaneous removal of NOx and PM from diesel vehicle exhaust

The objective of the present study is to investigate the simultaneous removal characteristics of NOx and PM over LNT/CDPF which the combination of an lean NOx trap (LNT) catalyst and a diesel particulate filter (DPF). LNT catalysts were prepared by an impregnation method, and then analyzed through TEM, BET surface area, and EDS to investigate their physical characteristics. It was found from experiments with the LNT catalysts that the NOx conversion of 2Pt20Ba/Al2O3 coated with cordierite substrate was 65% at 350°C, which demonstrated the highest de-NOx performance among other LNT catalysts. The NOx conversion of LNT(2Pt20Ba)/CDPF combined with LNT and CDPF was 49% at 370°C. When 5wt% cobalt was added to 2Pt20Ba/Al2O3 catalyst to improve the NOx conversion of the LNT/CDPF, the NOx conversion of the LNT(2Pt20Ba5Co)/CDPF was improved to 55% at 310°C. The initiation temperature for PM oxidation was 250°C, and LOT50 of the PM was 550°C over the LNT/CDPF. In particular, the rate of PM oxidation over the LNT/CDPF was higher than one over the bare DPF, which was not coated with LNT catalyst.

NH3 formation over a lean NOX trap (LNT) system: Effects of lean/rich cycle timing and temperature

A commercial lean NOX trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO2 was evaluated in this investigation. The effects of lean/rich cycle timing on the NOX, CO and C3H6 conversions and on the NH3 and N2O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NOX release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NOX conversions. For example, extending the rich period from 5 to 15s at 250°C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NOX conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450°C, NOX release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NOX conversions. For example, extending the rich period from 5 to 15s at 500°C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NOX conversion from 76 to 54%. The selectivities to NH3 and N2O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH3 and N2O yields were significantly affected by the cycle timing since they depend on the NOX conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NOX conversion also resulted in increased NH3 and N2O yields for a given temperature. Thus, concerted control of NH3 generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH3/NOX ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450°C. However, N2O yield also increased with NH3 yield under the same conditions.

Pd-Doped Perovskite: An Effective Catalyst for Removal of NOx from Lean-Burn Exhausts with High Sulfur Resistance

Herein, we report the Pd-doped perovskite La0.7Sr0.3CoO3 as an effective lean NOx trap (LNT) catalyst. This smart perovskite displays excellent NOx reduction activities for lean-burn exhausts (NOx conversion >90%, N-2 selectivity >90%) over a wide operating temperature range (275-400 degrees C), as well as an extremely high sulfur tolerance. Our results evidenced Pd dissolving into or segregating out of perovskite in lean-burn and fuel-rich atmospheres. The segregated metallic Pd from perovskite in fuel-rich atmospheres is crucial for obtaining these promising achievements. These findings provide a new possibility for the application of the Pd-based LNT catalysts.

Spectrokinetic Analysis of the NOx Storage Over a Pt–Ba/Al2O3 Lean NOx Trap Catalyst

In this work the kinetics of the (reactive) lean accumulation phase of the NOx storage-reduction process is described through a detailed kinetic model, involving both the gas-phase molecules and the adsorbed species. Kinetic data have been collected following a novel approach based on simultaneous operando spectroscopic measurements and on-line pulse reactor effluent analysis. To our knowledge, this is the first time the temporal evolutions of the concentration of both the surface and the gas species are used jointly to describe the kinetic of a transient catalytic process.

Enhanced Low Temperature NO x Reduction Performance Over Bimetallic Pt/Rh–BaO Lean NO x Trap Catalysts

The overall NSR operation was tested over a bimetallic Pt/Rh–BaO lean NOx trap (LNT) catalyst in the range of 473–673 K with simulated diesel exhausts and compared to monometallic 1 wt% Pt/BaO/γ-Al2O3 and 0.5 wt% Rh/BaO/γ-Al2O3 samples. The results showed the beneficial effect of the simultaneous presence of 0.5 wt% Pt and 0.25 wt% Rh on the catalytic performance under lean-burn conditions at low temperatures. It was observed that both Pt/BaO/γ-Al2O3 and Rh/BaO/γ-Al2O3, which both were mildly aged, have limited NOx reduction capacity at 473 K. However, combining Pt and Rh in the NOx storage catalyst assisted the NOx reduction process to occur at lower temperatures (473 K). One possible reason could be that the combined Pt and Rh sample was more resistant to aging. In addition, the NO2-TPD data showed that the presence of Rh into the Pt/BaO/γ-Al2O3 system has a considerable effect on the spill-over process of NOx, accelerating the release of NOx at lower temperatures. These results were in a good agreement with the observed higher rate of oxygen release of the bimetallic Pt/Rh catalyst, leaving a significant number of noble metal sites available for adsorption at lower temperatures than that of the monometallic Pt sample. The superior NSR performance of the bimetallic Pt/Rh/BaO/γ-Al2O3 catalyst under lean-burn conditions suggested the existence of synergetic promotion effect between the Pt and Rh components, increasing the NOx reduction efficiency in comparison with that of the monometallic Pt and Rh–BaO LNT catalysts.

Lean NOx Reduction with H2 and CO in Dual-Layer LNT–SCR Monolithic Catalysts: Impact of Ceria Loading

A series of monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of lean NOx trap (LNT) catalyst were synthesized for lean reduction of NOx (NO&NO2) with H2 and CO. The LNT catalyst exhibited a rather low NOx conversion below 250 °C due to CO inhibition. The top SCR layer comprising Cu/ZSM5 significantly increased the NOx conversion at low temperature by its reaction with NH3 formed during the regeneration phase. The addition of CeO2 to the LNT layer promoted the water gas shift reaction (CO + H2O ↔ H2 + CO2). The WGS reaction mitigated the CO inhibition and the generated H2 enhanced the low-temperature catalyst regeneration. The ceria addition decreased the performance at high temperatures due to increased oxidation of NH3. The ceria loading was optimized by applying a non-uniform axial profile. A dual-layer catalyst with an increasing ceria loading axial profile improved the performance over a wide (low and high) temperature range.

Nanofiber Alumina Supported Lean NOx Trap: Improved Sulfur Tolerance and NOx Reduction

NOx uptake and S resistance performance was studied over Pt–Ba and Pt–K lean NOx trap (LNT) catalysts, supported on “standard” Al2O3 and a nanofibrous Al2O3. The activity study was accompanied by TEM, XRD and H2 chemisorption characterization. The nanofibrous Al2O3-supported catalysts resulted in better performance. This included improved storage capacity, reduction efficiency during the regeneration phase, thermal stability and S-tolerance.

Reaction Pathways for Ammonia Formation on Lean NOx Trap/Reduction System: A Spectroscopic Infrared Investigation

This study is devoted to an operando study of Pt–Rh/Al2O3–BaO lean-NOx trap catalyst during the regeneration with H2/CO reaction mixture. Particular attention was paid to the influence of CO coexisting with H2 during the regeneration that can simulate the regeneration step by using reformate composed of CO and H2. In rich H2 mixture ammonia predominantly forms. As expected, strongly chemisorbed CO molecules over noble metals lower the efficiency of the trap at 150 °C. Successive hydrogenation of N atoms to ammonia predominates in our conditions. However, the comparison of the outlet gas composition with infrared spectral features also suggests a minor participation of isocyanate species (NCO) as possible intermediates in the production of ammonia especially for long regeneration duration in the absence of water. Interestingly, ammonia formation as reducing agent for the selective reduction of NO can stimulate practical applications for further coupling lean-NOx trap with SCR catalysts.

NOX storage and reduction over a perovskite-based lean NOX trap catalyst

Typical lean NOX trap (LNT) catalysts contain Pt, which catalyzes NO oxidation and NOX reduction, and an alkali or alkaline earth material for NOX storage via nitrate formation. The catalyst is operated in a cyclic mode, with one phase of the cycle under oxidizing conditions where NOX is trapped, and a second phase, which is reductant-rich relative to O2, where stored NOX is reduced to N2. A recently developed catalyst uses a perovskite material as part of the LNT formulation for the oxidation reactions thereby eliminating the need for Pt. Pd and Rh are still added, to accommodate hydrocarbon oxidation and NO reduction, respectively. NO oxidation kinetics over the fully formulated and bare perovskite material were determined, with NO, O2 and NO2 orders being at or near 1, 1 and −1, respectively for both samples. The fully formulated sample, which contains Ba supported on the perovskite, was evaluated in terms of NOX trapping ability and NOX reduction as a function of temperature and reduction phase properties. Trapping and overall performance increased with temperature to 375°C, primarily due to improved NO oxidation, as NO2 is more readily trapped, or better diffusion of nitrates away from the initial trapping sites or into the Ba particles. At higher temperatures nitrate stability decreased, thus decreasing the trapping ability. At these higher temperatures, a more significant amount of unreduced NOX formed during the reduction phase, primarily due to nitrate instability and decomposition and the relative rates of the NOX and oxygen storage (OS) component reduction reactions. Most of the chemistry observed was similar to that observed over Pt-based LNT catalysts. However, there were some distinct differences, including a stronger nitrate diffusion resistance at low temperature, a more significant reductant-induced nitrate decomposition reaction and nitrate inhibition of OSC reduction at the onset of the regeneration phase.

The different NOxtrap performance on ceria and barium/ceria containing LNT catalysts below 200 °C

The low-temperature NOx storage efficiency of Ce-containing lean NOx trap catalysts was investigated using temperature programmed adsorption. The incorporation of ceria either as a NOx storage phase or as the support for barium oxides improves the NOx storage capacity from 200 °C to 350 °C. However, as temperature increases from 100 °C to 150 °C, the rate of NOx storage slows down over the catalysts containing ceria as a storage phase, while this phenomenon is less obvious if ceria is used as the support for barium oxides. By investigating the individual components of catalysts using NO-TPD, we found that the NOx release was mainly from ceria. Based on the in situ DRIFT data, we conclude that nitrite species are formed on ceria during the NOx storage at low temperatures, and the nitrite species can be activated and transformed to nitrate species upon further oxidation, accompanied by NO release; on the other hand, NO release occurs weakly on barium/ceria as the nitrites are more stable on the more basic Ba phase than on the ceria. In addition, the dispersion of barium species on ceria is higher than that on γ-Al2O3, resulting in the formation of a larger amount of basic sites. The addition of barium onto ceria not only improves the stability of ad-NOx species but also positively impacts the amounts of NOx trapped.

Lean NOx reduction on LNT-SCR dual-layer catalysts by H2 and CO

Lean reduction of NOx (NO & NO2) by H2 and CO was conducted over monolithic catalysts consisting of a selective catalytic reduction (SCR) catalyst layer deposited on top of a lean NOx trap (LNT) catalyst. An increase in the CO/H2 ratio decreased the cycle-averaged NOx conversion for the ceria-free LNT catalyst. CO poisoning was especially significant below 250°C. The low-temperature NOx reduction was increased either by use of an LNT-SCR dual-layer catalyst or deposition of CeO2 on the LNT catalyst. However, the ceria decreased the high-temperature reductive conversion of NOx due to promotion of the undesired NH3 oxidation. Ceria zoning enhanced the monolith NOx conversion. Downstream loading of ceria led to the highest NOx reduction at both low- and high- temperatures due to the beneficial interaction of the ceria and H2. The low-temperature NOx conversion of an aged dual-layer catalyst could be increased by a higher SCR catalyst loading. However, at high temperatures NOx reduction was independent of the SCR loading. The ratio of the lean to rich feed duration and the total cycle time were optimized to improve the NOx conversion in a temperature range from 150 to 400°C. The highest cycle-averaged NOx conversion was obtained with a 30s:5s lean-rich cycle containing 1.25% total reductant for all CO/H2 ratios for a lean feed containing 500ppm NO and 5% O2.

Advantages of syngas for the regeneration of NOx trap system investigated with operando IR measurements

The regeneration of Lean NOx Trap catalyst (Pt–Rh/BaO–Al2O3) using H2+CO was followed with operando IR spectroscopy methodology. The role of H2 was investigated from 2 to 20vol.% H2. The increase of H2 partial pressure allowed a subsequent decrease of the regeneration duration. The quantity of NH3 formed during the regeneration was related to H2 quantity which suggests that NH3 formation is governed by the supply of H2. The reactivity of nitrate was found slower than that of nitrite species towards hydrogen at 250°C. CO addition strongly inhibited ammonia production at 150°C due to accumulation of carbonate species at the surface of the catalyst. Above 250°C, two processes coexisted for the reduction of NOx into ammonia in the presence of H2+CO mixture: the successive hydrogenation of N atoms from NO dissociation and the hydrolysis of isocyanate species evidenced by IR.